In many safety critical applications, there is a requirement that the electronic systems thereof operate with a high degree of reliability. In order to achieve a particular level of reliability, it may be necessary or desirable to incorporate a built in test capability, capable of detecting key faults, into such electronic systems. A built in test approach requires that the electronic system includes functionality that enables it to self test critical functions.
In many electronic systems, decoupling capacitors are used to stabilise a voltage at a node of an electronic circuit by, for example, suppressing power supply noise and/or switching noise. External (discrete) decoupling capacitors are frequently connected at power supply and voltage reference connections (e.g. analog ground) of integrated circuits to decouple them from other parts of the circuit. A properly connected and functional decoupling capacitor may be critical for the proper operation of the electronic system. Capacitors may also be required in other critical applications, for example in timing and filter circuits.
In broad terms, failures of a capacitor may be split into failures of the capacitor itself and failures of electrical connections between the capacitor and the other circuit elements. Both short circuit and open circuit failures are possible, for example as a result of dielectric breakdown and breaking of an electrical connection respectively.
An example of a potentially safety critical system is an accelerometer with an electronic readout circuit, which may be used to trigger the airbags or other safety devices (for example seatbelt tensioners) of a vehicle. It is important that an accelerometer used in such a context is highly reliable and, as mentioned above, this may require that the circuits associated with the accelerometer are able to detect and report fault conditions that prevent the proper operation thereof. Where a decoupling capacitor is used to reduce noise on the analog ground for such a system, it may be connected to the readout application specific integrated circuit (ASIC) via solder connections and wire bonds. Disconnection of the decoupling capacitor may result in unacceptable levels of noise on the output of the accelerometer, or unacceptable coupling between the supply voltage and the output voltage.
JP2000230960 describes an apparatus for checking for defective connector contacts during the manufacture of an electronic module. The apparatus is operable to inject a current pulse and check whether the resulting disturbance to the capacitor voltage falls within an expected range. The apparatus described does not appear to be suitable for incorporation into an electronic circuit and so can only be used during manufacture or conducting subsequent servicing operations. It is not thought to be suitable for use in detecting failures whilst the electronic module is in use.
There is a need for a method and apparatus that enables built in testing of capacitors. A method and apparatus that provides a reliable indication with the minimal potential of a false indication of a fault is preferable. Further, an on-line or continuous built in test method is preferred that enables the electronic system, of which the capacitor is a part, to continue to function properly while capacitor testing is under self test.